The invention relates to an inductive sensor arrangement for detecting a ferromagnetic or non-ferrous electrically conducting object buried in a surrounding medium. The inductive sensor arrangement comprises at least one field coil for generating a penetrating alternating magnetic flux in the medium and at least one sense coil for sensing magnetic flux field disturbances caused by the ferromagnetic or non-ferrous electrically conducting object. The invention further relates to a method, and its application in a hand-held machine tool, for detecting a ferromagnetic or non-ferrous electrically conducting object hidden in a surrounding medium by use of the inventive inductive sensor arrangement.
Generally, metal detectors measure the change of a chosen parameter as an operator sweeps a sensing head across a surface of a medium wherein a disturbing piece of a metal could be hidden. The parameter may be a capacitance, an inductance or any other physical parameter that would provide a distinction of one material from another.
Specifically, detectors may be used to find reinforcing metal bars (hereinafter xe2x80x9crebarsxe2x80x9d), usually consisting of a ferrous material embedded in a media such as concrete, brick, plaster and the like. There are detectors on the market that can fulfill this requirement but the accurate ones must be swept across the surface of the medium, e.g. concrete. By such a xe2x80x9csweeping movementxe2x80x9d, it is possible to determine the position and the (length) direction of the hidden object, e.g. the rebar, from the received response signals. The metal coverage area may either be determined automatically using a rather complicated system or manually. In a manual determination, it is usual practice to manually mark the coverage and the direction of the rebar on the surface of the medium. Such manual scanning and determination requires not only time but also a specific skill and knowledge of the user or operator.
U.S. Pat. No. 5,729,143 describes a metal detector including a receive coil and a transmit coil arranged in parallel overlapping winding planes and connected in an inductive bride. Such an arrangement is a typical example of a metal detector that requires the specific skill and knowledge of the operator for interpreting the signal response.
Another type of apparatus for detecting metallic objects in a non-metallic material is described in U.S. Pat. No. 2,546,771. This detecting apparatus comprises a pair of series connected secondary coils positioned with their radii in substantially parallel planes adjacent to such material to be introduced into a detecting gap between two primary windings, respectively, diametrically encircling only one of each of the secondary coils. The primary coils are energized by an alternating current with such polarities that their magnetic fluxes are additive through such material. The secondary coils are connected in series bucking relation to produce a zero resultant output voltage across the secondary coils when the magnetic field of the primary coil is undistorted in the absence of any metallic object eventually hidden in the material probe. This known detecting apparatus, however, due to its physical dimensions and arrangement of gapped primary coils, is not suitable for detecting ferromagnetic or non-ferrous electrically conducting objects in a surrounding medium.
EP 0 366 221 AZ discloses a buried metal detection device comprising yoke-connected coils that are rotated to produce an external alternating electromagnetic field to be aligned in different directions, and monitoring variations in the energy supply drawn by the coil as an indication for the presence or non-presence of a metallic object in a surrounding medium.
EP 1 092 988 A2 discloses an inductive sensor arrangement for detecting metal objects hidden in a surrounding medium comprising a field coil pair for generating an alternating magnetic flux by a sequential excitation with an AC-current, and a pair of sense coils, respectively mounted inside each associated field coil in an orientation to the axes of each of the field coils such that essentially no voltage is induced in the sense coils in an environment free of a metallic object. If a metallic object comes into the vicinity of the inductive sensor, four characteristic voltage value sets are produced in the sense coil pair, wherein the voltage sets are processed by an algorithm for defining a position and a distinction criterion with respect to the hidden metallic object. This sensor arrangement has the advantage of a single point measurement resulting in an accurate position discrimination for a hidden metallic object like a rebar in concrete. Such an inductive sensor, however, has the limitation that reliable detection of a ferrous object like a rebar is only possible within angles of xc2x160xc2x0 or less, usually only xc2x145xc2x0. To make the detector functional over a full 360xc2x0 range such that arbitrary rebar angles may be detected, the EP-document also proposes that a second pair of field/sense coils is placed at 90xc2x0 positions with respect to the first pair of field/sense coils to cover the orthogonal xc2x145xc2x0 range. Such a four field/sense coil arrangement, however, if intended as an add-on rebar detection tool for a hand-held appliance such as a drill hammer, becomes dimensionally bulky and relatively expensive.
DE 196 48 833 A1 describes a device for detecting and identifying hidden objects like plastic mines in a ground. This device comprises two side-by-side arranged sensor coils that are operated at different excitation frequencies. Depending on various physical properties of the hidden object such as electrical conductivity, permeability etc. the impedance of a receiver coil arranged in an overlapping configuration of the two sensor coils is modified differently depending on the respective material properties. Again, the scanning of a specific ground area and the interpretation of the receive signals requires experience and skill.
It is an object of the present invention to provide an inductive sensor arrangement and a method for detecting ferromagnetic or non-ferrous electrically conducting objects like rebars hidden in a medium which, on the one hand, is capable of full 360xc2x0 range detection and a depth determination and, on the other hand, allows a compact and small sensor design.
An inductive sensor arrangement, in accordance with the invention, comprises three field coils arranged in essentially the same geometric plane and adjacently positioned at defined angular distances from each other in relation to and around a central axis orthogonal to the plane; and three sense coils, one sense coil being respectively mounted inside each field coil in an orientation to the axis of the respectively associated field coil such that essentially no voltage is induced in the sense coil from the respectively associated field coil in an environment free of a ferromagnetic or non-ferrous electrically conducting object or material. Preferably, the three field/sense coil pairs are positioned at equal angular distances approximately 120xc2x0 from each other, and the axes of the sense coils are respectively arranged orthogonally with respect to the axis of their associated field coil.
A method for detecting a ferromagnetic or non-ferrous electrically conducting object hidden in a surrounding medium by the inductive sensor arrangement, in accordance with the invention, comprises the steps of excitation of the three field coils with defined current ramps sequentially applied to each of said field coils to produce a changing magnetic flux penetrating the medium originating from three different physical positions corresponding to the field coils arrangement, collecting nine distinct output voltages from the three sense coils, i.e.
while the first field coil is excited by the defined current ramp, a first output voltage is acquired from the associated first sense coil and second and third output voltages from the remaining sense coils, respectively,
while the second field coil is excited by the next defined current ramp, a fourth output voltage is acquired from the associated second sense coil and fifth and sixth voltages from the remaining first and third sense coils, respectively,
while the third field coil is excited by the next following defined current ramp, a seventh output voltage is acquired from the associated third sense coil and eighth and ninth voltages from the remaining first and second sense coils, respectively, and
gaining a present or non-present criterion for a ferromagnetic or non-ferrous electrically conducting object by algorithmic processing of the nine distinct output voltages.
Preferably, the algorithmic processing steps are performed sequentially with respect to a set of one voltage values for each of the nine output voltages sensed by the three sense coils during one excitation current ramp cycle supplied to each of the three field coils.
Using the sensor arrangement of the invention it becomes possible to accurately locate an electrically conducting (a metallic) or ferromagnetic object, in particular a rebar, from a single point measurement. A determination of the hidden object not only with regard to a YES/NO-answer, but also with regard to its depth in the medium below a measuring spot on the surface becomes possible with a low-cost sensor device of small spacial dimensions. The perfect functioning of the inductive sensor arrangement according to the invention is surprising insofar as the respectively oblique 120xc2x0 arrangement of the field/sense coil pairs was originally believed to be impossible since the shape of the detection curves have to be similar for a correct decision, where the rebar is. The arrangement of the sensor of the invention surprisingly shows that three field/sense coil pairs have some other significant advantages over the double pair orthogonal arrangement of field/sense coil pairs of the prior art.
For the user of a hand-held tool machine like a drill hammer in the four field/sense coil pairs of the prior art, the magnetic components make a complete circle around the sensor head. Such an arrangement make it difficult to see any desired drilling marks. In the three field/sense coil pairs according to the invention, there is sufficient room to provide for features in the external molding to increase the vision.
Further, the three field/sense coil pairs arrangement needs significantly less battery power and consequently gives a longer battery operating life time. Also, the three coil pairs sensor can be manufactured considerably cheaper and the weight of the completed hand-held tool machine is lighter, which is important at the drill tip, in particular, since it reduces the time on the operator""s wrist.
A further advantage of the three coil pairs arrangement, according to the present invention vis-a-vis the known four coil pairs sensors, is that a simple algorithm can be used requiring only the S-curve response signal and the hump-curve response signals which will be explained below in further detail. In contrast, in a four coil pairs arrangement opposite pairs of sense coils and their respective response signals are processed separately to cover an angular range of xc2x145xc2x0 and the second pair of field/sense coils, i.e. the opposite sense coils, are required to cover the orthogonal xc2x145xc2x0 range.
A sensor head and measuring unit according to the invention is simple to use, reliable and because of a one point measurement requirement also allows the operator to work in confined spaces.